Electrical lighting fixtures often involve the use of ballasts, and it is known practice to provide protection from overheating by including “thermal protectors” in the circuitry in order to break the flow of current to the ballasts when the thermal protector reaches a predetermined temperature. The prior art includes ballasts with either internal or external thermal protectors (i.e., internal or external with respect to the coil) positioned to interrupt the input power flowing to the primary coil of the ballast. In many prior art ballasts, thermal protectors are positioned at or near what are believed to be the most likely locations for insulation degradation, i.e., potential insulation failure points.
Ballasts for high-intensity-discharge (HID) lamps are often deemed to require thermal protection for various reasons:
In the past, a predominant reason for thermal protection of ballasts has been the possibility of insulation failures occurring within a ballast due to the expansion and contraction of the core and coil caused by the on-and-off operation over an extended period of time. Ballast overheating is related to degradation of insulation on windings due to thermal stresses. Insulation degradation can frequently develop at or near points where input voltage taps are embedded within the layers of the primary coil. Ballest manufacturers have been particularly concerned, therefore, about protecting ballasts from such aging effects.
Another known reason for providing thermal protection for a ballast is the overheating which can occur by virtue of the increased power requirements of lamps as they age, and it is this source of overheating which is the primary motivation for this invention. More specifically, as HID lamps age, two basic changes take place: One change involves the wearing (by sublimation) of the electrodes, causing the gap between the electrodes to increase slightly, raising the voltage and thereby the power required. Another change involves chemical variations and contaminations in the gaseous mixture within the lamp arc, which also tend to raise the power requirements of the lamp. Ballasts are designed to provide increasing power as demanded and, as the power rises, the steady-state temperature of the ballast also rises. In order to protect ballasts from failing catastrophically at the ends of the useful lives of certain HID lamps, it is desirable to break the flow of current at a point in time when the temperature has risen above a level deemed acceptable. These concerns are particularly important with respect to pulse-start HID lamps.
In the prior art, thermal protection issues appear to have been dealt with primarily from the ballast manufacturer's viewpoint, as described above. Thermally-protected ballasts of the prior art most typically place thermal protectors in series with the primary coil in order to interrupt the flow of power when the temperature rises above pre-set limits.
The ballast industry has developed ballasts with multiple input voltage taps on the primary coils to address the requirements for multiple voltages in HID lighting applications, thereby providing ballasts which can be adapted to a variety of HID lighting situations. (HID lighting in, e.g., the United States operates at one of several voltages, including 120V, 208V, 240V, 277V and 480V.) However, providing such flexibility in a thermally-protected ballast translates into higher assembly or installation costs for the electrical lighting manufacturer or installer, and, in both cases, creates opportunities for assembly or installation errors. Among other things, in such situations the presence of multiple input taps can be a source of costly wiring errors caused by improper placement of a thermal protector (on the wrong input line) in particular lighting applications.
There is a need for an improved thermally-protected ballast particularly suitable to protect against problems caused by end-of-life operation of HID lamps, particularly the increasingly common pulse-start HID lamps which are now frequently used in place of metal halide probe-start HID lamps of older design. While the old-style HID lamps typically reach a point where they can no longer operate and are replaced, the pulse-start HID lamps more typically will continue to operate longer and thus require higher power levels over time. Without appropriate protection, this entails greater risks of catastrophic failures.
Such need is particularly applicable to ballasts with multiple input voltage taps. There is also a need for low-cost, reliably assembled, and easily usable thermally-protected ballasts, and for devices to hold thermal protectors in place to ensure good thermal contact.